Optical device having mesas

ABSTRACT

An optical device and method for fabricating an optical device. The optical device comprising: a semiconductor material comprising an active layer configured to emit light when an electrical current is applied to the device and/or to generate an electrical current when light is incident on the active layer, wherein the semiconductor material comprises a first surface and an opposed second surface, from which light is emitted from and/or received by the device, and wherein the first surface defines a first structure comprising the active layer and configured to reflect light emitted from the active layer toward the second surface and/or to reflect light received by the device toward the active layer, and the second surface defines a second structure configured to permit light incident on the second surface at an angle outside a critical angle range to the planar normal to pass therethrough.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/EP2013/067187, filed Aug. 16, 2013,which claims the benefit of EP1215632.9, filed Sep. 3, 2012, both ofwhich are incorporated by reference.

TECHNICAL FIELD

The invention relates to an optical device. Specifically, the inventionrelates to, but is not limited to, an optical device with efficientcoupling of light between the device and the environment. In specificexemplary devices, the invention relates to, but is not limited to, alight emitting diode (LED) having an improved extraction efficiency(EE).

BACKGROUND OF THE INVENTION

LEDs convert electrical energy into optical energy. In semiconductorLEDs, light is usually generated through recombination of electrons,originating from a first doped semiconductor layer, and holes,originating from a second doped semiconductor layer. In some infra-redemitting semiconductor materials light can be generated by electronintersub-band transitions rather than electron-hole transitions. Thearea where the main light generation takes place is referred to hereinas the active layer, or more generally as the active region.

A major challenge in the field of LEDs is to extract as much of theemitted light as possible from the semiconductor material into thesurrounding medium, which may be air.

There are a number of problems with traditional cuboid-shaped LEDsincluding: 1) most light emitted from the active layer is outside theangle of total internal reflection, which leads to long path lengths forrays; and 2) they suffer from high absorption due to increased pathlengths

An approach used to improve LED performance is called chip shaping.Higher EEs are possible with this approach. However, it does noteliminate the long path lengths within the LED chip, nor the requirementfor an external mirror. Also, the technique is less suitable to thewidely used gallium nitride (GaN) based LEDs. The reason for this isthat the sapphire and silicon carbide (SiC) substrates commonly used inGaN-based LED chips are very hard materials and are very difficult toshape mechanically, for example, with a dicing saw. In LEDs comprisingthese materials, it seems not to be a practical solution to shape thewhole chip.

Another approach to achieving a high EE is to provide an array of“micro-LEDs”, thus keeping the average path length within the deviceshort. Such arrangements are described in U.S. Pat. No. 6,410,940 andU.S. Pat. No. 6,410,942.

Total internal reflection is a common problem for LED devices, as therefractive index of the substrate materials used is typically muchgreater than air, which typically surrounds the LED. This allows lightto escape from only a very narrow range of escape angles (or criticalangle range) around the normal to the exit surface. The escape angle ismore limited as the difference in refractive index of the materialsbecomes larger. For typical LED devices, in the simplest case, onlyapproximately 10% of the generated light is extracted. The designchallenge is to create a structure which allows more light to beextracted into the air.

Packaging techniques may also be used to enhance the EE of LEDs. Forexample, the extraction efficiency can be increased by profiling anencapsulant over a packaged semiconductor LED device. The encapsulantmay be epoxy or phosphor crudely shaped like a lens. This can improvethe EE by around 5 to 10%. Additional 90° lenses placed on a packagedLED device can be used to improve the EE of the device by around 10 to20%.

Processing approaches that user planar fabrication techniques arepreferred by the LED industry as they are compatible high volumemanufacturing. Any modification to an LED design should use suchtechniques.

Surface roughening of the semiconductor surface still maintains a planarconcept, and these designs are still considered to be ‘planar’ innature. Optimised surface roughening can currently achieve over 50% EE.However, there are limitations to this planar surface rougheningapproach. Firstly, the design still results in multiple internalreflections before the light escapes. This results in losses, especiallyreflections on the contact electrodes, even if the reflections arerelatively efficient. Secondly, the design typically includes mirrors aspart of the electrical contact on the planar surface to enhance thelight extraction. This requires additional manufacturing processes.

Sapphire-based semiconductor devices (i.e. a Sapphire substrate) formthe basis for the production of the majority of high production volumesand lower power devices. For the larger and higher power devices, thedesign requirements mean the material properties of the sapphiresubstrate become more limiting. Thicker substrates are advantageous toallow more light to escape from the sidewalls of the device. Sapphirehas a poor thermal conductivity, so the device becomes hotter as thesubstrate becomes thicker. In terms of thermal management, thelimitations of the sapphire substrate can be overcome by removing thesemiconductor layer from the sapphire substrate and bonding to anotherthermally and electrically conducting substrate—a process known aswafer-lift off. This can provide a good thermal management solution. Amirror on the bonding interface is typically employed to improve thelight extraction efficiency especially when combined with surfaceroughening on the exit surface.

In some cases an additional bonded substrate can be fabricated at anglesother than 90° such that the reflections on these angled surfacesenhances the light extraction. This can enable an EE of up to around65%.

A micro-LED structure is proposed in WO 2004/097947 (U.S. Pat. No.7,518,149) with a high EE because of its shape. Such a micro-LED 100 isshown in FIG. 1, wherein a substrate 102 has a semiconductor epitaxiallayer 104 located on it. The epitaxial layer 104 is shaped into a mesa106. An active (or light emitting) layer 108 is enclosed in the mesastructure 106. The mesa 106 has a truncated top, on a side opposed to alight transmitting or emitting face 110. The mesa 106 also has anear-parabolic shape to form a reflective enclosure for light generatedor detected within the device. The arrows 112 show how light emittedfrom the active layer 108 is reflected off the walls of the mesa 106toward the light exiting surface 110 at an angle sufficient for it toescape the LED device 100 (i.e. within the angle of total internalreflection).

SUMMARY OF THE INVENTION

The present applicant has identified that achieving an increase in EE ishindered by total internal reflection at the surfaces of thesemiconductor, and the applicant's observations on this and relatedissues are summarised below. In particular, as a result of research intothe micro-LED structures set out, for example, in WO 2004/097947 (U.S.Pat. No. 7,518,149), the applicant has observed the following.

The increased EE is achieved by creating a reflector structure (e.g. themesa) such that a greater portion of the generated light is directed tothe exit surface at an incident angle less than the critical angle tothe normal, at which light cannot escape.

The design of the mesa profile is an approximation of a parabolicdesign. The reflector design takes advantage of the high reflection oflight seen at the junction between high and low refractive indexmaterials. That is, light incident on the sidewalls of the mesa isreflected toward the light exiting surface.

The technology requires a flat and polished exiting surface to achievethe highest efficiencies. Any surface roughness can act to modify theincident angle outside the critical angle range causing internalreflection not transmission.

The improvement in EE that can be achieved is essentially a factor oftwo. In terms of the overall light extraction, this allows approximately20% of the generated light to escape the LED device, when compared withthe typical EE of an LED of 10% mentioned above. Other extractionmethods (e.g., surface roughening) can currently achieve EEs of greaterthan 50%.

To form the mesa structure, the epitaxial layer of the LED devices mustbe etched away. This significantly reduces the area of the active layeravailable to generate light. This causes a decrease in the lightgenerated per unit area for the same applied current density. Increasingthe current to offset this area reduction does not compensate the areareduction, as light output is not linear with increasing current.

The combined increase in EE and reduction in the area that light isgenerated mean that, even in the best case, an increase in the devicesurface area of at least a factor of three is necessary to generate thesame optical flux, under equivalent electrical forward drive conditions.

The applicant has identified the desirability of improving theperformance of micro-LED devices. Specifically, the applicant hasidentified a desirability to increase both the EE (for example, by morethan a factor of two) and total optical flux per unit area extractedfrom each micro LED device (for example, by more than a factor ofthree).

According to the invention in a first aspect, there is provided anoptical device comprising: a semiconductor material comprising an activelayer configured to emit light when an electrical current is applied tothe device and/or to generate an electrical current when light isincident on the active layer, wherein the semiconductor materialcomprises a first surface and an opposed second surface, at which lightis emitted from and/or received by the device, and wherein the firstsurface defines a first structure comprising the active layer andconfigured to reflect light emitted from the active layer toward thesecond surface and/or to reflect light received by the device toward theactive layer, and the second surface defines a second structureconfigured to permit light incident on the second surface at an angleoutside a critical angle range to the planar normal to passtherethrough.

Optionally, the semiconductor material is an epitaxial layer. That is,the first structure and the second structure are formed on opposedsurfaces of the epitaxial layer that comprises the active layer of thedevice.

Optionally, the epitaxial layer may be mounted on one of a GaN wafer, aSilicon wafer, a sapphire wafer, a silicon carbide wafer, a coppersubstrate, a ceramic substrate, a glass wafer, a GaA wafer or any othersubstrate generally used in the industry

Optionally, the first and/or second structures comprise mesas withtruncated tops.

Optionally, the truncated tops of the first and/or second mesas aresubstantially circular.

Optionally, the first and/or second mesas have a circular footprint andthe footprint of the mesa of the second structure has a larger diameterthan the footprint of the mesa of the first structure.

Optionally, the truncated tops of the mesa of the second structure has alarger diameter than the truncated top of the mesa of the firststructure.

Optionally, the first and/or second structures are generally parabolicin cross section. The second structure may have a planar top portion andsidewalls curved inwardly toward the first structure. Generally, thesecond structure may describe a hemisphere with the top section removed.The second structure may be shaped to cooperate with the first structureto increase the amount of light incident on the second surface able toescape the device.

Optionally, the first and second structures are co-aligned.

Optionally, the second structure covers at least the lateral areacovered by the first structure.

Optionally, the second surface defines a Fresnel lens.

Optionally, the optical device further comprises a reflective layerdeposited on the first surface to internally reflect light within thedevice.

Optionally, the reflective layer is electrically conducting and forms anelectrical contact for the device.

Optionally, the optical device comprises one of a micro-LED and aphotodiode.

According to the invention in a second aspect, there is provided anarray of optical devices comprising a plurality of optical devices asdescribed above.

Optionally, the optical devices are formed from a single piece ofsemiconductor material.

Optionally, a pitch between the optical devices in the array is at leastthe diameter of the truncated top of the mesa of the second structure.

According to the invention in a third aspect, there is provided a methodof making an optical device, the method comprising: providing asemiconductor material comprising an active layer configured to emitlight when an electrical current is applied to the device and/or togenerate an electrical current when light is incident on the activelayer, wherein the semiconductor material comprises a first surface andan opposed second surface, at which light is emitted from and/orreceived by the device; shaping the first surface to define a firststructure comprising the active layer and configured to reflect lightemitted from the active layer toward the second surface and/or toreflect light received by the device toward the active layer; andshaping the second surface to define a second structure configured topermit light incident on the second surface at an angle outside acritical angle range to the planar normal to pass therethrough.

Optionally, the semiconductor material is an epitaxial layer on asubstrate, and the method may further comprise a wafer lift-offprocedure comprising removing the epitaxial layer from the substrate,such that the second surface is accessible.

The wafer lift off procedure may further comprise planarizing the firstsurface and attaching a second substrate on the planarized firstsurface.

An optical device is proposed that comprises a semiconductor materialhaving a first surface and an opposed second surface, with both surfacesbeing structured to increase an extraction efficiency of the device. Thefirst surface is shaped to form a first surface structure which acts asa reflective enclosure for light generated or detected by an activeregion within or associated with the semiconductor material of the firstsurface structure. The second surface is shaped to form a second surfacestructure, with the second surface structure corresponding generally(e.g. in shape and/or position) with the first surface structure.

The first and second surfaces may be considered to be structured so asboth to be non-planar.

The second surface structure is co-aligned with the first surfacestructure.

The first and second surface structures may both form protrusions orconcave surface elements when viewed from outside the material.

The second surface structure may have a shape which generally mirrorsthat of the first surface structure, and/or which is generally aninverted form of the first surface structure.

The first surface structure may be generally dome shaped. The secondsurface structure may be generally dome shaped.

The first surface structure may be shaped as a mesa with a truncatedtop. The mesa may have a generally parabolic shape. The second surfacestructure may be shaped as a sub-mesa with a truncated top,corresponding generally in shape to the mesa of the first surfacestructure.

The second surface structure may cover at least the lateral area coveredby the first surface structure.

The optical device may be a light emitting device or a light receivingdevice. In this respect, while light emitting devices (e.g. LEDs)convert electrical energy into optical energy, and although the bulk ofthis disclosure relates to such light emitting devices, it will bereadily understood that methods and apparatus disclosed herein can alsobe applied to the reverse operation, i.e. to light receiving devices(e.g. photodiodes or PDs) that convert optical energy into electricalenergy.

The second surface structure may be provided with a shape which takesaccount of the shape of the first surface structure so as to maximisethe light in use which is incident upon the second surface within acritical angle range, the light having emanated from the active layer(in the case of a light emitting device) or falling upon the activelayer (in the case of a light receiving device).

The active layer may be a light emitting or receiving region, where thedevice is a light emitting device or a light receiving devicerespectively.

The second surface may be a light output or input surface, where thedevice is a light emitting device or a light receiving devicerespectively.

The reflective enclosure of the first structure may be adapted toreflect light from the active layer in a forward direction (towards thesecond surface structure) or reflect light received from a forwarddirection (from the second surface structure) to the active layer, wherethe device is a light emitting device or a light receiving devicerespectively.

The first and second surface structures may be formed from the samesemiconductor material.

There is also proposed a method of making an optical device, comprisingproviding a semiconductor material having a first surface and an opposedsecond surface, shaping the first surface to form a first surfacestructure to act as a reflective enclosure for light generated ordetected by an active region within or associated with the semiconductormaterial of the first surface structure, shaping the second surface toform a second surface structure, with the second surface structurecorresponding generally with the first surface structure.

The method may comprise forming an array of such optical devices on acontinuous substrate.

The method may further comprise the step of separating the opticaldevices of the array into smaller arrays or single devices. Separatingmay comprise mounting the substrate to a carrier, and using an etchprocess to separate the devices.

Other features of the invention may include:

1. The formation of a light emitting device where both surfaces of theepitaxial layer are non-planar or structured.

2. Both electrical contacts are on one side of the patterned surfaces.Light is extracted from the side without the electrical contacts.

3. The shaping of optical profiles on both surfaces of a semiconductorLED device into the semiconducting material produces high opticalextraction efficiency greater than can be achieved by a single sideoptical profile and a planar surface (unstructured surface).

4. The diameters of the optical profiles are different. The diameter ofthe sub-mesa is greater than the micro-LED mesa on the opposite surface.

5. Registration of the optical structures is important to achieve theoptimum combined benefit. In particular, the central axis of bothstructures may be the coincident.

6. The thickness of the semiconductor material is dependent on the pitchof the first structure and second structure combination and targetoptical parameters defined.

7. The manufacture can be extended through the semiconductor materialisolating each first structure/second structure providing the electrodeside is well mounted. In this way the mesa structures become discreteunits.

8. No metal electrode schemes are required on the exit surface. Theelectrodes can be defined on one side of the semiconductor material.

9. No shadowing due to metal tracking. In this respect, the formation ofthe second structure may be larger than the first structure. Theformation of the first structure naturally exposes the n-GaNsemiconductor material. The combination of the material exposure and theplanar surface provides the opportunity to create both contacts from oneside and allow light to exit unhindered from the other.

By way of summary, micro-LED structures described in WO 2004/097947(U.S. Pat. No. 7,518,149) have an improved extraction efficiency (EE)because of their shape. The light generated within the mesa has a higherprobability of escape through the opposite planar exit surfaceespecially when the near parabolic mesa has a high aspect ratio. Theresulting extracted light has also a more directional beam. Themicro-LED relies on a flat and smooth or polished exit surface from thesemiconductor material to achieve this improvement. Whilst the micro-LEDtechnology can achieve high EE, the design cannot achieve a high totaloptical flux per unit area. This is because the formation of themicro-LED results in only a small area that generates light relative tothe overall device size.

Methods and apparatus disclosed herein describe a further implementationof the micro-LED where the EE is further increased by advantageouslystructuring the previously flat exit surface by creation of a sub-mesa(second structure). In WO 2004/097947 (U.S. Pat. No. 7,518,149) it isdisclosed that the sapphire and silicon carbide (SiC) substratescommonly used in GaN-based LED-chips are both very hard materials andvery difficult to shape mechanically, for example with a dicing saw. Inthese material systems it seems not to be a practical solution to shapethe whole chip.

Where the structure of the semiconductor material is free-standing anddoes not include any of these hard materials it is possible to structureboth sides of the semiconductor material. The sub-mesa is defined at aprecise position relative to the mesa, i.e. the central axis of the mesaand sub-mesa are ideally common. In very general terms, this can beconsidered a dual lens system. This design allows the lightadvantageously directed by the first structure toward the second surfaceto exit as previously described on a planar surface, whilst alsoallowing light at greater angles than the perpendicular to be alsoextracted through the exit surface shaped as the sub-mesa.

The combination of the micro-LED structure and the exit surface sub-mesastructure create a highly efficient design that can be realised even ifthe aspect ratio of the micro-LED mesa is reduced. This is advantageousto achieve high electrical wall plug efficiency (WPE) as well as highoptical efficiency because the area of the semiconductor generatinglight remains relatively high. In this case, the current densities usedto drive the light generating regions can be similar to conventional LEDdevices. Overall, a high total optical flux, high extraction efficiencyand relatively narrow beam profile can be achieved that are improvementsin existing device designs where at least one surface is stillessentially planar.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a micro-ED device having anactive layer enclosed in a mesa structure;

FIGS. 2a and 2b show two cross-sectional views of an optical devicestructure;

FIGS. 3a and 3b show sectional views of an array of optical structures;

FIG. 4 shows a method for fabricating an optical device;

FIGS. 5a-5e show structures arising during a fabrication process;

FIG. 6 shows a Fresnel lens second-mesa;

FIGS. 7 and 8 show modelling data.

DETAILED DESCRIPTION

It is noted that the following description refers predominantly to LEDdevices but the invention should not be limited to such. The inventionapplies equally to light receiving devices, such as photodiodes.Further, in this specification the term “light” will be used in thesense that it is used in optical systems to mean not just visible light,but also electromagnetic radiation having a wavelength outside that ofthe visible range.

For micro-LED structures having an active layer enclosed in a mesastructure, the applicant has identified a relationship between thegeometrical dimensions of the mesa, the total amount of light generatedand the amount of light extracted. In the case of a micro-LED, thedirectionality (beam profile), is directly linked to the materialproperties and the difference in refractive indices between the materialof the LED and the surrounding medium. The applicant has thereforeestablished that the main areas to improve in a micro-LED design are thetotal optical flux extracted and the total optical flux per unit area ofthe device.

After research and development activities, the applicant has identifiedthe following.

The main losses in such a micro-LED device are due to the physical sizeof the active layer. This is not a point source, but a broad area on ornear the truncated top surface of the mesa. As the ratio of height todiameter (H/D ratio) for the mesa increases, the efficiency of amicro-LED structure increases, but the total light output decreases perunit area as the actual area of the active layer is smaller. As the H/Dratio decreases, the electrode area increases and more light isgenerated as less active layer is removed to form the mesa structure. Inthis case, more light is generated, but not as efficiently extracted bythe micro-LED geometry. This is the main area where further advantagescan be achieved with low H/D ratios.

The term “quasi-collimated” is used herein to define the light confinedwithin the critical escape angle of an LED device. Light generated inthe active layer must exit either: (a) directly through an exit facewithout reflection on the mesa sidewall; (b) via a single reflection onthe mesa sidewall resulting in an incident angle to the exit face withinthe critical angle range; or (c) following multiple reflections withinthe mesa structure. This is shown in FIG. 1.

Light that is generated by the active layer but is not extracted throughthe exit surface generally either undergoes multiple reflections on themesa sidewalls and is incident on the exit surface at an angle that isnot transmitted, or is emitted from the active layer at an angle that isnot incident on the mesa sidewall and at an angle greater than thecritical angle to the exit surface. Multiple reflections can also resultin a broader incident angle on the exit surface than single reflections;these are typically transverse around the central axis of the mesastructure and are not sufficiently re-directed by the micro-LED design.In other cases the generated light is typically lost through internalreflections in the device and exits eventually from some other surfaceon the semiconductor structure or is absorbed within the semiconductormaterial.

The probability of light generated in the active layer escaping throughthe exit face decreases as the distance from the mesa central axis ofthe point in the active layer at which the light is generated increases.This is because a single reflection on the mesa is less likely to bereflected to be incident on the exit surface within the critical anglerange, and the mesa design is not able to re-direct the incident lightat the extreme positions. If the whole of the top of the mesa isdesigned to generate light (in the case of low H/D ratio designs), thenmore light is produced at the extreme positions from the mesa centralaxis and this is not effectively redirected. There is little additionalbenefit to this design and the structure is therefore less efficient.

Previous micro-LED structures, in which the light emitting layer isdisposed on a mesa offer a performance (efficiency) improvement over apurely planar device through an interaction of EE, radiation profile(beam angle) at the expense of total optical flux per unit area.

Methods and apparatus disclosed herein make significant improvements intotal optical flux per unit area. Generally, this may be achieved bydefining an additional complementary structure to the micro-LED on anopposite surface (or light emitting surface) of the semiconductormaterial of the LED. This will be referred to as a second structure, orsub-mesa.

FIGS. 2a and 2b show an optical device, which is a micro-LED structure200 comprising a semiconductor material 201 having a first surface 202defining a first structure 203. In the exemplary device of FIGS. 2a and2b , the first structure 203 is a mesa with a truncated top. The firstmesa 203 comprises an active layer 204. A second surface 206 of thedevice 200 is opposed to the first surface 202. The second surfacedefines a second structure 208, which in the exemplary device of FIGS.2a and 2b is a corresponding sub-mesa, or second mesa. The secondsurface 206 also defines a surface at which light may be emitted fromthe device 200. It is noted that in a photodiode device, light isreceived at the second surface 206.

The semiconductor material 201 is formed by a first semiconductor 209comprising the semiconductor material 201 above the active region 204,and a second semiconductor 211 comprising the semiconductor material 201below the active region 204. The first and second semiconductors 209,211 are oppositely doped, that is, one is p doped and the other is ndoped. Together, the first and second semiconductors 209, 211 form anepitaxial layer of the device 200. Therefore, methods and apparatusdisclosed herein provide an optical device 200 in which opposed surfaces202, 206 of the epitaxial layer are shaped and profiled.

As can be seen from FIG. 2b , the first mesa 203 is generally parabolicin cross section and has a circular footprint. The second mesa 208 isflat topped with the sides bending inwards toward the semiconductormaterial so as to ensure that as many of the light rays coming from theactive layer 204 hit these bended sides at an angle within a criticalrange of angles 210 so that the light passes into the surrounding air.That is, the sidewalls of the second mesa 208 are curved such that theypresent a normal that is always directed toward the active layer 204.This is the case even at points on the second surface 206 that are notaxially aligned with a point on the active layer 204.

In the exemplary device of FIGS. 2a and 2b , the mesas 203, 208 arebroadly hemispherical with truncated tops, which are circular. The mesa203 of the first structure has a circular footprint that is smaller thanthe circular footprint of the mesa 208 of the second structure. Themesas are also co-aligned such that the centre line of the first mesa203 is coincident with the centre line of the second mesa 208.

The arrows shown in FIG. 2a illustrate light emitted from the activelayer 204 of the LED device in a plurality of directions. Some light isemitted from the active layer 204 directly toward the second surface.Other light is reflected from the sidewalls of the first mesa 203 atvarious other angles toward the second surface 206. Light arriving atthe second surface 206 is at an angle to a planar normal, which is adirection normal to a planar surface of the first and/or secondstructures 203, 208. In the exemplary devices of FIGS. 2a and 2b , theplanar surfaces are defined by the truncated tops of the mesas 203, 208and the planar normal is therefore at right angles to those surfaces.

A critical range of angles 210 is defined and only light incident on thesecond surface 206 within that critical range of angles 210 is allowedto pass through. The critical range of angles 210 defines a cone inthree dimensions. In a device having a completely planar second surface,only light received from the active layer 204, either directly or byreflection, that is within the critical angle range 210 exits thedevices. However, as shown in FIG. 2a , by shaping the second surface206 to have convex curved shape, light that is outside the criticalrange of angles 210 to the planar normal is allowed to pass through thesecond surface 206. For example, the light rays represented by arrows212 a and 212 b are internally reflected from the first mesa 203 at anangle outside the critical range of angles 210 to the planar normal.However, because that light is incident on a section of the secondsurface 206 that is curved, they are within a critical range of anglesto the normal at that point of the surface. Therefore, the lightrepresented by arrows 212 a and 212 b is allowed to pass through thesecond surface 206 and is not reflected.

FIGS. 3a and 3b show an array of optical devices 300 comprising aplurality of optical devices 200, which in this exemplary apparatus aremicro-LED structures. Each micro-LED structure 200 comprises a firstmesa 203 comprising an active layer 204, and a corresponding second mesa208, as shown in FIGS. 2a and 2 b.

The diameters of the first mesa 203 and second mesa 208 are defined asthe diameter of the footprint thereof. The array of micro-LEDs of FIGS.3a and 3b has a pitch equivalent to the second mesa 208 diameter. Thisresults in benefits in the EE due to the second mesa 208 profile,despite the increased pitch between the devices. The adjacent micro-LEDdevices do not significantly interfere with each other even under theseconditions.

An optical device 200 design according to exemplary methods andapparatus and including the second mesa 208, aims to maintain the lightextraction advantages already offered by the micro-LED structure (an EEof approximately 20% of the generated light) for: (a) direct (noreflections); and (b) single/multiple reflections on/in the mesa.

In addition, methods and apparatus disclosed having the second mesa 208design aligned to the first mesa 203 allow at least a portion of themultiple or transverse reflected light to exit the second surface 206.That is, the second mesa 208 is configured to allow light that isincident on the second surface 206 outside the critical range of angles210 to the planar normal to pass through. This can be achieved bycreating an additional profile to the second surface 206 of thesemiconductor device 200. The non-planar profile is mainly formed in theregions where light would be incident on the exit surface at anglesgreater than the critical range of angles 210 and so would be internallyreflected by a planar second surface.

The design of such a second surface 206 is complex for the same reasonsas the micro-LED mesa definition. The broad emitting area is far frombeing a point source and so there must be a compromise structure to theideal lens design for a point source. The basic critical angle rangecone 210 still has a range of incident angles from 0 to θ (the criticalangle), with the design of the second mesa 208 profile being toeffectively modify the ‘normal’ or θ=0 away from the planar normal to bemore advantageous for the incident light. The present applicant hasappreciated that this can most effectively (perhaps only) be achieved bydefining the second mesa 208 profile into the original semiconductormaterial. Any additional material would still have a planar interfaceand so the design would not generally achieve a main aim.

The defined geometrical profile can be considered as a second mesa 208,as there is a strong correlation between the first mesa structure 203and the corresponding surface profile on the second surface 206. Theresult is a variation of a two lens system, where the first mesa 203defines the first lens, and the second mesa 208 the second lens.

The relationship of the second mesa 208 geometry to the first mesastructure 203 is important for increased extraction efficiency. In someembodiments an equivalent Fresnel lens may be used instead of the secondmesa 208. It is noted that it has been found that this does not providethe equivalent enhancement of the extraction efficiency. Similarly, aFresnel lens cannot readily be included as an additional separatestructure as this would create a planar interface resulting in furtherinternal reflections.

The combination of the first mesa 203 and the second mesa 208 act topreserve some control of the beam directionality. This is better than aLambertian beam profile, but not as well defined as having a first mesa203 alone. This is a design compromise with LED structures as disclosedherein. Generally, a compromise is to be made between high extractionefficiency or a narrow radiation profile.

There is a strong spatial correlation between the first mesa 203 andsecond mesa 208 designs. For this reason the thickness of thesemiconductor material should also be specified and taken into account.With the first mesa 203 alone, the light extraction is independent ofthe material thickness. This presents an additional confinement todefine the performance optimisation to those already presented by knowntechnologies.

In this regard, where the first mesa 203/second mesa 208 combinationform a discrete pixel in an array of optical devices, the semiconductorthickness is limited to a value determined by the pitch of the array (orvice versa)

On the other hand, where the first mesa 203/second mesa 208 combinationis used in a large area cluster, there is a compromise to be made in thecluster design layout. For high total optical flux per unit area, theH/D ratio of the micro-LED design should be as small as possible, andthe pitch as small as possible. These values will then define theoptimum semiconductor material thickness. For high efficiency and anarrow beam profile of the exiting light, the choice of micro-LED H/D,pitch is defined by the second mesa 208 diameter and the optimummaterial thickness must be specifically designed in each case.

In an example where the micro-LED design ratio is H/D=0.5, this allows alarge electrode area. The maximum ratio of the single emitter area istherefore approximately 25% based on the area of the sub-mesa. This canbe used to define an acceptable range for the semiconductor thickness asa multiple of the array pitch.

In exemplary methods and apparatus, the footprint of the second mesa 208will be larger than the footprint of the first mesa 203 due to thenature of the design requirements. The intersections of the pixels of anarray according to a methods and apparatus disclosed herein are definedby the geometry of the second mesa 208 not the mesa structure. In aexemplary methods and apparatus, there will therefore be space betweenadjacent micro-LEDs.

In designing an LED structure array according to an embodiment, designoptimisation may be necessary for the second mesa 208 intersection.

The design can be optimised in other ways. For example, for an efficientwall plug efficiency (WPE) design, maintain the highest p-contactelectrode area, priority for the micro-LED design to have as low a H/Dratio as possible. For a narrow radiation profile, increase the H/Dratio of the micro-LED and modify the design of the sub-mesa to achievethe desired radiation profile.

The fabrication of the second mesa 208 would typically use the samefabrication techniques as known micro-LED mesa techniques. In thisrespect, the etch method and profile fabrication techniques can beconsidered as being common to both mesa and second mesa 208.Registration to the micro-LED is possible through the transparentsemiconductor substrate.

To manufacture the first mesa 203/second mesa 208 combination, bothstructures can be fabricated into the epitaxial layer. In the case wherethe micro-LED diameter is small, potentially the epitaxial layer canbecome very thin. Where the pitch of the mesas is large, the thicknessof the epitaxial layer can be greater than the height of the first mesa203 (H) and the depth of the second mesa 208 (M); in this case there isno impact on the mechanical stability of the semiconductor material.Where the pitch of the first mesa 203/second mesa 208 combination is assmall as possible, the epitaxial layer thickness is likely to berelatively thin (in the order of 2-10 micrometers). When the height ofthe first mesa 203 and depth of the second mesa 208 become the same asthe thickness of the epitaxial layer, there is the opportunity toisolate each first mesa 203/second mesa 208 combination; in this case,the mechanical stability of the overall device can be provided as partof the electrical contacts.

The optical devices in the array can be operated as clusters (i.e., allworking in unison to create more light) for applications such lighting.The optical devices in the array can be operated as individuallyaddressable arrays (i.e., each pixel individually switchable) forapplications such as printing. The first side structures (mesa height,mesa diameter, location of epitaxial layer within the mesa structure,exact parabolic structure, pitch between pixels) and the second sidestructures (mesa/lens shape, mesa/lens diameter, mesa/lens pitch,mesa/lens height) can be optimized to prioritise the optical deviceperformance (e.g., lumens/watt, efficiency, lumens per unit area,optical beam half/angle, collimation). If the optical device emits at UVwavelengths (200 nm to 385 nm), the distance light travels within thedie before emerging is critical to the device's efficiency. Thesemiconductor/substrate itself absorbs the light (i.e. EE is very low)and this is a major technical challenge in such devices. Therefore asolution that does not restrict the light bouncing around within thechip does not increase EE. The methods and apparatus disclosed maximizeEE in UV generating LEDs.

As mentioned in WO 2004/097947 (U.S. Pat. No. 7,518,149), the design andfabrication techniques are suitable for all semiconductor materialssystems where the light can be extracted through a transparent bottom orbase material. Examples of such systems are: GaN/GaN, GaN/Silicon,GaN/sapphire, GaN/SiC, InGaAs/GaAs, and InGaAsP/InP. An embodiment ofthe invention applies to either p-type doped semiconductor above theactive layer, and n-type doped semiconductor below the active layer, orwith n-type doped semiconductor above the active layer, and p-type dopedsemiconductor below the active layer. If the manufacturing process useswafer life-off approaches, or this process is introduced into theproduction process, both surfaces of the substrate are convenientlyaccessible during the fabrication process so that the mesa and sub-mesamay both be formed during the fabrication process.

Electrical contacts can both be formed on the first mesa 203 side of thesemiconductor material. This is advantageous in that the light extractedthrough the exit surface is not shadowed by the tracking pattern.

One possible manufacturing process flow is as follows:

A. Micro-LED (e.g. first mesa 203) fabrication, as described in WO2004/097947 (U.S. Pat. No. 7,518,149)

B. N-metal contact formation

C. Planarization (optional)

D. Final mount assembly

E. Substrate removal

(D&E being wafer lift-off)

F. Second mesa 208 fabrication (similar to that for the micro-LED)

-   -   a. Etch mask definition    -   b. GaN (semiconductor) Etch    -   c. Mask removal

Methods and apparatus disclosed herein may be based on, and may buildon, the methods and apparatus of WO 2004/097947 (U.S. Pat. No.7,518,149), and in view of this the entire content of WO 2004/097947(U.S. Pat. No. 7,518,149) is hereby incorporated by reference, in amanner sufficient to provide support for any claims included in thepresent application now or in the future and to provide basis for anyamendments to the present application.

A method of fabricating an optical device 200 is described withreference to FIGS. 4 and 5 a-5 e. A semiconductor material 201comprising oppositely doped first and second semiconductors 209, 211 isprovided 400. The semiconductor material forms an epitaxial layer andcomprises an active layer 204 configured to emit light when anelectrical current is applied to the device and/or to generate anelectrical current when light is incident on the active layer. Theepitaxial layer is positioned on a substrate 500. The substrate 500 maycomprise a semiconductor material and, although it is not shown this wayin FIGS. 5a-e for clarity, is typically one or two orders of magnitudethicker than the epitaxial layer. Typically, the thickness of theepitaxial layer is in a range from 2-10 micrometers and the substrate isin the order of a few hundred micrometers.

The first surface 202 is shaped 402 to form the first structure (ormesa) 203. This may be done by etching and methods suitable are set outin WO 2004/097947. FIG. 5a shows schematically the resulting structureafter the first mesa 203 has been etched.

The first surface 202 is planarized 404 (Using BCB—bisbenzocyclobutaneor similar) to fill in the regions surrounding the first mesa 203.Typically, a plurality of optical devices 200 are fabricated on a waferand planarization fills the gaps between the mesas 203 to provide a flatsurface. This is shown in FIG. 5 b.

A wafer lift off process 406, 408 is undertaken. This removes 406 thesubstrate 500 from the second surface 206 of the epitaxial layer toexpose the second surface for shaping. This is shown in FIG. 5c . Then,a further substrate (or carrier) 502 is attached 408 on the planarizedfirst surface 202. The new substrate 502 provides structural rigidity tothe device and is shown in FIG. 5 d.

The second surface is shaped 410 to form the second structure (or mesa)208. This may be done by etching using standard fabrication methods. Theresulting optical device is shown in FIG. 5 e.

For clarity, many of the steps are not shown in FIG. 4, although thesewill be known to the skilled person. Similarly, many further featureswill be required in FIG. 5e to make the device 200 operational, butthese are not shown, for clarity purposes.

The structuring of the second surface structure 208 is enabled if theepitaxial layer is removed and attached to a carrier wafer (wafer liftoff technologies) that exposes the inner side of the epitaxial layerenabling further processing/etching using ‘normal’ or typical LEDprocessing approaches (e.g., planar fabrication techniques). Such waferlift off processing techniques are a core component of GaN on Si waferprocessing, making the methods and apparatus disclosed specificallycompatible with GaN on Si LED technologies.

In modelling, the methods and apparatus disclosed herein have shown anEE of around 85%, as shown in FIGS. 5 and 6.

Whilst specific embodiments of the invention are described hereinbefore,it will be appreciated that a number of modifications and alterationsmay be made thereto without departing from the scope of the invention.

What is claimed is:
 1. An optical device comprising: a semiconductormaterial comprising an active layer configured to emit light when anelectrical current is applied to the device and/or to generate anelectrical current when light is incident on the active layer, whereinthe semiconductor material comprises a first surface and an opposedsecond surface, at which light is emitted from and/or received by thedevice, wherein the first surface defines a first structure comprisingthe active layer and configured to reflect light emitted from the activelayer toward the second surface and/or to reflect light received by thedevice toward the active layer, and the second surface defines a secondstructure configured to permit light incident on the second surface atan angle outside a critical angle range to the planar normal to passtherethrough, the first and second structures comprising mesas withtruncated tops, and wherein the mesas have a circular footprint and thefootprint of the mesa of the second structure has a larger diameter thanthe footprint of the mesa of the first structure.
 2. An optical deviceaccording to claim 1, wherein the semiconductor material is an epitaxiallayer of the device.
 3. An optical device according to claim 1, whereinthe truncated tops of the mesas are substantially circular.
 4. Anoptical device according to claim 1, wherein the truncated top of themesa of the second structure has a larger diameter than the truncatedtop of the mesa of the first structure.
 5. An optical device accordingto claim 1, wherein the first and/or second structures are generallyparabolic in cross section.
 6. An optical device according to claim 1,wherein the first and second structures are co-aligned.
 7. An opticaldevice according to claim 1, wherein the second structure covers atleast a lateral area covered by the first structure.
 8. An opticaldevice according to claim 1, wherein the second surface defines aFresnel lens.
 9. An optical device according to claim 1, furthercomprising a reflective layer deposited on the first surface tointernally reflect light within the device.
 10. An optical deviceaccording to claim 9, wherein the reflective layer is electricallyconducting and forms an electrical contact for the device.
 11. Anoptical device according to claim 1, wherein the optical devicecomprises one of a micro-LED and a photodiode.
 12. An array of opticaldevices comprising a plurality of optical devices, each optical devicecomprising a semiconductor material comprising an active layerconfigured to emit light when an electrical current is applied to thedevice and/or to generate an electrical current when light is incidenton the active layer, wherein the semiconductor material comprises afirst surface and an opposed second surface, at which light is emittedfrom and/or received by the device, wherein the first surface defines afirst structure comprising the active layer and configured to reflectlight emitted from the active layer toward the second surface and/or toreflect light received by the device toward the active layer, and thesecond surface defines a second structure configured to permit lightincident on the second surface at an angle outside a critical anglerange to the planar normal to pass therethrough, the first and secondstructures comprising mesas with truncated tops, and wherein the mesashave a circular footprint and the footprint of the mesa of the secondstructure has a larger diameter than the footprint of the mesa of thefirst structure.
 13. An array of optical devices according to claim 12,wherein the truncated tops of the mesa of the second structure has alarger diameter than the truncated top of the mesa of the firststructure.
 14. An array of optical devices according to claim 12,wherein the optical devices are formed from a single piece ofsemiconductor material positioned on a substrate.
 15. An array ofoptical devices according to claim 12, wherein a pitch between theoptical devices in the array is at least the diameter of the truncatedtop of the mesa of the second structure.
 16. A method of making anoptical device, the method comprising: providing a semiconductormaterial comprising an active layer configured to emit light when anelectrical current is applied to the device and/or to generate anelectrical current when light is incident on the active layer, whereinthe semiconductor material comprises a first surface and an opposedsecond surface, from which light is emitted from and/or received by thedevice; shaping the first surface to define a first structure comprisingthe active layer and configured to reflect light emitted from the activelayer toward the second surface and/or to reflect light received by thedevice toward the active layer; and shaping the second surface to definea second structure configured to permit light incident on the secondsurface at an angle outside a critical angle range to the planar normalto pass therethrough, the first and second structures comprising mesaswith truncated tops, and wherein the mesas have a circular footprint andthe footprint of the mesa of the second structure has a larger diameterthan the footprint of the mesa of the first structure.
 17. A methodaccording to claim 16, wherein the truncated tops of the mesa of thesecond structure has a larger diameter than the truncated top of themesa of the first structure.
 18. A method according to claim 16, whereinthe semiconductor material is an epitaxial layer positioned on asubstrate, and wherein the method further comprises a wafer lift-offprocedure comprising removing the epitaxial layer from the substrate,such that the second surface is accessible.
 19. A method according toclaim 18, wherein the wafer lift off procedure further comprisesplanarizing the first surface and forming a second substrate on theplanarized first surface.
 20. An optical device comprising: asemiconductor material comprising an active layer configured to emitlight when an electrical current is applied to the device and/or togenerate an electrical current when light is incident on the activelayer, wherein the semiconductor material comprises a first surface andan opposed second surface, at which light is emitted from and/orreceived by the device, wherein the first surface defines a firststructure comprising the active layer and configured to reflect lightemitted from the active layer toward the second surface and/or toreflect light received by the device toward the active layer, and thesecond surface defines a second structure configured to permit lightincident on the second surface at an angle outside a critical anglerange to the planar normal to pass therethrough, the first and secondstructures comprising mesas with truncated tops, and wherein thetruncated top of the mesa of the second structure has a larger diameterthan the truncated top of the mesa of the first structure.